While polycarbonate resin is tough and excellent in impact strength or electrical properties and also in dimensional stability, it has such drawbacks as high melting viscosity, poor moldability, thickness dependency observed in impact strength, poor chemical resistance as exemplified by occurrence of cracks when contacted with an aromatic solvent or gasoline. For example, at 23.degree. C., when the thickness exceeds 1/4 inch, brittle fracture is caused, and, even in a test piece of 1/8 inch thickness, impact strength tends to lower as temperatures go down. Thus, polycarbonate resin is inevitably limited in the field of its application.
For overcoming these drawbacks, a variety of improvements have been proposed. For example, in JPA S56(1981)-45946 and JPA S56(1981)-45947, an improvement in the impact strength of aromatic polycarbonate resin is seen by incorporating an acrylic impact modifier thereinto. According to this method, the impact strength is indeed improved, but, in colored molded articles, non-uniformity of color appearance which is, so called, color shading or pearlessence is caused. This phenomenon is especially remarkable at the site where the resin composition receives a high shear rate as in the gate region in the process of molding. Therefore, uses of such colored molded articles as above are inevitably restricted without applying further treatments such as surface painting. And, as mentioned above, at the site where the resin composition receives a high shear rate in the process of molding, delamination is often observed, leading, in some instances, to molded articles which can hardly be put to practical uses. In JPA S56(1981)-28234, there is disclosed a thermoplastic resin composition from which a molded article excellent in impact strength can be prepared by incorporating a multi-stage graft copolymer containing dienic rubber into aromatic polycarbonate resin. This is still not free from remarkable appearance of color shading or pearlessence on the molded articles.
On the other hand, in JPB S61(1986)-9982, it is disclosed that the impact strength of polycarbonate resin can be improved without losing its transparency by incorporating a multi-layered polymer consisting of a polymer of the first layer prepared by polymerization of an aromatic vinyl monomer, a polymer of the second layer prepared by polymerization of an alkyl acrylate monomer whose alkyl group has 1 to 8 carbon atoms and a third layer polymer, whose glass transition temperature is not lower than 50.degree. C., prepared by polymerization of an aromatic vinyl monomer, into the polycarbonate resin together with a homopolymer or copolymer of an aromatic vinyl monomer.
According to this method, the transparency of the resulting resin composition is indeed not lost, but the object of this method is clearly different from that aiming at preventing a resin composition from appearance of color shading. Besides, in the impact strength, no reference is made to thickness dependence or impact strength at low temperature, and the result of improvement in impact strength is far from being satisfactory.
And, for overcoming such drawbacks observed in polycarbonate resin as less satisfactory moldability and chemical resistance, various proposals have been made. For example, in JPB S36(1961)-14035, an improvement of aromatic polycarbonate resin in its chemical resistance by incorporating polyethylene terephthalate resin thereinto is proposed. Also, in JPA S48(1973)-54160, an improvement of aromatic polycarbonate resin in its surface hardness and chemical resistance by incorporating polybutylene terephthalate resin thereinto is proposed. But, in these resin compositions, impact strength is not satisfactory.
Furthermore, in JPB S55(1980)-9435, a method of improving aromatic polycarbonate in its impact strength by incorporation of aromatic polyester and butadienic elastomer, and, in JPB S62(1987)-37671, a method of improving aromatic polycarbonate resin in its impact strength by incorporation of polyester resin and acrylic elastomer, are respectively proposed. According to these methods, impact strength is indeed improved, but, on colored molded articles, color shading or pearlessence appears, and, especially at the site where the resin composition receives a high shear rate in the process of molding, these phenomena are conspicuous. And, at the site where the resin composition receives a high shear rate in the process of molding, delamination is often observed, resulting sometimes in providing molded articles which cannot be put to practical use.
For solving such problems as involved in color shading possibly caused in the production of colored articles, in JPB H1(1989)-34463, there is proposed incorporation of a multi-layered polymer consisting of a first layer polymer prepared by polymerization of a styrenic monomer, a second layer polymer prepared by polymerization of an alkylacrylate monomer whose alkyl group has 1 to 8 carbon atoms and a third layer polymer prepared by polymerization of an alkylmethacrylate monomer whose alkyl group has 1 to 8 carbon atoms into a mixture of colored polycarbonate resin and polyester resin. However, even by this method, the problem of color shading is not solved sufficiently, and the impact strength of the resultant product at low temperatures is not satisfactory.
On the other hand, polyester resin is excellent in chemical resistance, thermal resistance, weatherability and moldability, but low in impact strength. For improving the impact strength, various proposals have been made so far. For example, in JPA S52(1977)-74652 and JPA H2(1990)-191614, a method of incorporating a core-shell elastomer containing epoxy group into polyester resin, and, in JPA S52(1977)-150466, a method of incorporating a core-shell elastomer containing no epoxy group into polyester resin. Even in colored molded articles prepared from these compositions, however, color shading or pearlessence is still observed, and the color appearance is still problematic.